Promoting the Activity of Arginine Decarboxylase and Ornithine Decarboxylase by Ethylene and Its Significance to the Control of Abscission in Citrus Leaf Explants

It is well accepted that auxin delays and ethylene accelerates abscission (5). During the abscission process cells in the abscission zone (AZ) pass from a juvenile stage, characterized by cell division and enlargement, to a senescence stage leading to cell wall degradation and organ separation. Extensive evidence links polyamines (PA) to the control of cell division perceives them as “antisenescence” agents (1,4,8), while ethylene, is considered a senescence-promoting hormone (1). The antisenescence properties of exogenously applied PA may result from both an inhibition of ethylene biosynthesis and promotion of endogenous PA biosynthesis (8). Many studies dealt with the interactions between auxin and ethylene in the control of abscission (5). However, the biosynthetic linkage between PA and ethylene via S-adenosylmethionine as a common intermediate, and the fact that PA directly antagonize many ethylene-mediated responses (1,8), raised the question whether PA are involved in the control of abscission. The linkage between PA and ethylene was previously studied both at the biosynthetic and at the physiological levels, and mainly in relation to senescence and fruit ripening (1). Most of the reports dealt with the effects of exogenous PA on several aspects of ethylene production and biosynthesis, and only a few examined how exogenous ethylene interferes with PA levels and biosynthesis (1,8). The interactions between plant hormones and PA can be summarized by saying that in general, plant hormones that promote growth induce an increase in the biosynthesis of PA while plant hormones that retard growth decrease PA biosynthesis (for detailed references see reviews 1,4,8). Exogenous ethylene was reported to decrease the activity of arginine decarboxylase (ADC) and S-adenosylmethionine decarboxylase, and to increase the activity of ornithine decarboxylase (ODC) and lysine decarboxylase (2,6). In the following we refer to a study on the effects of ethylene on abscission of citrus leaf explants; PA biosynthesis was examined to establish the kind of relationships existing between the two regulators in the process of abscission.